This invention relates to steam-methane reformer furnaces, and more particularly to the application of a convection-heated transition duct pre-reformer in the design of a steam-methane reformer furnace.
Hydrocarbon reforming furnaces are widely used to make synthesis gas containing hydrogen for hydrogen plants, methanol plants, ammonia plants and the like. The capacity of these plants is often limited by the amount of hydrogen that can be supplied to them, which in turn is limited by the capacity of the reformer furnace(s). The capacity of the reforming furnace can be limited by several factors, including the firing rate, the heat transfer rate in the radiant section, the heat transfer surface area (i.e., the number of catalyst-filled tubes) available in the radiant section, and the size of the radiant section of the reforming furnace. It would be very desirable to be able to increase the capacity of the reforming furnace without increasing the size or heat transfer area of the radiant section of the furnace.
The life of the radiant section and its components generally depends on the temperatures and firing rates in the radiant section. The higher the operating temperature of the radiant section is, generally the shorter the useful life. The more frequently the radiant section must be taken off line for maintenance, the less profitable the unit becomes. It would be very desirable to reduce the firing rate of the radiant section and lengthen its useful life and the time between maintenance shutdowns, without diminishing hydrogen capacity of the furnace while it is in operation.
The overall efficiency and/or total fuel firing requirement of a reformer furnace is largely determined by the required absorbed duty of the catalyst-filled radiant tubes. It would be very desirable to reduce the duty requirement of the radiant tubes so that the radiant section firing rate can be reduced and/or recovery of waste heat can be accomplished more economically.
The tubes in the radiant section of a conventional reforming furnace are generally filled with a catalyst such as nickel on an alumina support. Great care must ordinarily be taken to minimize the formation of coke on the catalyst, as well as the introduction of catalyst-poisoning contaminants in the feed stream supplied to the tubes. Coke formation generally occurs at the entry of the hot feedstock into the tubes, before sufficient hydrogen is present in the gas to inhibit coke formation, and for this reason, each tube is filled with smaller-sized catalyst particles on top of a larger catalyst shape that fills the remainder of each tube. The smaller catalyst particles provide a relatively high ratio of surface area to volume, and a lower void fraction, which minimize the film temperature to inhibit coke formation that can otherwise reduce catalyst activity and foul the tubes. However, the smaller catalyst particle zone in each tube greatly complicates the catalyst loading into the tubes, and significantly increases the pressure drop through the tubes. The catalyst at the tube inlet is also more susceptible to deactivate in the event catalyst poisons are fed into the tubes.
One traditional method for increasing the hydrogen production rate has focused on the employment of a high-nickel, high activity catalyst to improve conversion in an adiabatic (unheated) pre-reformer. However, the high activity catalyst is expensive to use, and can be overly sensitive to coke formation and catalyst poisoning.
U.S. Pat. No 3,094,391 to Mader discloses convection-heated hydrocarbon reforming in parallel with radiant-heated reforming. The influents to the two different reforming reaction tubes are split apart from a common feedstock supply, and the effluents from the convection- and radiant-heated tubes are mixed together. The flue gas from the radiant section is passed longitudinally along, as opposed to transversely across, the tubes in the convection-heated section, which can be finned to improve heat transfer. This configuration is said to improve the efficiency of the furnace, but is vulnerable to the detrimental effects of differences between the heat transfer distribution among the catalyst tubes in the convection-heated reaction zone. Moreover, the Mader design can also result in different conversion rates between the convection-heated and radiant-heated reaction zones, and provides no means for compensating for any possible differences in heat transfer and conversion rates.
U.S. Pat. No 4,959,079 to Grotz et al. discloses convection-heated reforming in the upper or entry portion of radiant-heated catalyst tubes by directing the flue gases through a convection-heated channel through which the catalyst tubes extend. This upper portion of each catalyst tube is designed with an extended surface such as studs or longitudinal fins to improve heat transfer. Again, this design can result in different heat transfer and conversion rates, and provides no means for compensating for these differences. Furthermore, this design does not change the number of the radiant-heated tubes, and therefore has limited advantage.
The present invention uses a convection-heated reaction zone in series with the radiant-heated reaction zone so that the convection-heated reaction zone functions as an upstream pre-reformer. To avoid the effects in differences in heat transfer and conversion rates between tubes in the convection-heated pre-reformer, partially reformed effluent from the convection-heated reaction zone is preferably collected and distributed to the radiant-heated catalyst tubes. The convection-heated pre-reformer tubes absorb up to 15 or 20 percent of the required reforming load of the reformer furnace, reducing the reformer furnace firing or tube count by a corresponding proportion. The upstream pre-reformer can reduce the overall size of the reformer. This is because, for a given capacity, the radiant section can be 15-20 percent smaller and the convection-heated pre-reformer tubes can be located in the transition duct without altering the footprint or connecting flanges of the transition duct. The pre-reformer can use relatively large diameter tubes to add relatively little pressure drop. Conventional nickel catalyst can be used; high activity catalyst is not required in the radiant section or in the pre-reformer. The pre-reformer can act as a guard for the radiant-section catalyst, assuring a relatively high hydrogen content to avoid coke formation or catalyst poisoning in the radiant section. A uniform relatively large catalyst size can be used in the radiant section to simplify catalyst loading and minimize the overall pressure drop. The benefits of the pre-reformer can be applied to reduce the cost of either or both the convection section and the combustion air preheater.
In one aspect, the present invention provides a reformer, comprising: a fired radiant section, a transition section and a convection section. The radiant section produces a hot flue gas. The transition section receives the hot flue gas from the radiant section and partially cools a flow of the flue gas through the transition section. The convection section further cools the partially cooled flue gas from the transition section. A feed preheat exchanger is disposed in the convection section for preheating a hydrocarbon feed stream. A convection-heated pre-reformer is located in the transition section for partially reforming the preheated hydrocarbon feed stream from the preheat exchanger to form a hydrogen-containing, partially reformed feed stream. The pre-reformer comprises a plurality of catalyst-filled tubes disposed transversely to the flow of the flue gas through the transition section. A manifold is provided for distributing the partially reformed feed stream into a plurality of catalyst-filled tubes disposed in the radiant section for reforming the feed stream to form a hydrogen-rich synthesis gas. A manifold is provided for recovering the hydrogen-rich synthesis gas from the catalyst-filled tubes in the radiant section.
In another aspect, the present invention provides a steam reforming process for producing synthesis gas from a furnace comprising radiant, transition and convection sections. The process comprises the steps of: (a) firing the radiant section to produce a hot flue gas; (b) passing the hot flue gas serially from the radiant section through the transition and convection sections to recover heat from the flue gas; (c) passing a hydrocarbon feed stream through a preheat exchanger in the convection section to preheat the hydrocarbon feed stream in indirect heat exchange with the flue gas; (d) passing the preheated hydrocarbon feed stream from the preheat exchanger into a convection-heated pre-reformer comprising a plurality of catalyst-filled tubes disposed in the transition section transverse to a flow of the flue gas therethrough to form a hydrogen-containing, partially reformed feed stream; (e) distributing the partially reformed feed stream into a plurality of catalyst-filled tubes disposed in the radiant section for reforming the feed stream to form a hydrogen-rich synthesis gas; and (f) recovering the hydrogen-rich synthesis gas from the catalyst-filled tubes in the radiant section.
In a further aspect, the present invention provides a method for retrofitting a steam reformer that comprises (1) a fired radiant section for heating catalyst-filled reforming tubes and producing a hot flue gas, (2) a convection section for recovering heat from the flue gas, (3) an adiabatic transition section for passing the flue gas from the radiant section to the convection section, (4) a feed preheat coil in the convection section for preheating a hydrocarbon feed stream, and (5) a manifold for distributing the preheated hydrocarbon feed stream from the feed preheat coil into the reforming tubes. The method comprises the steps of: (a) installing a convection-heated pre-reformer in the transition section comprising a plurality of catalyst-filled tubes transverse to a flow of the flue gas therethrough for forming a hydrogen-containing, partially reformed feed stream; (b) installing a first line or lines for passing the preheated hydrocarbon feed stream from the feed preheat coil into the pre-reformer; and (c) installing a second line or lines for passing the partially reformed feed stream from the pre-reformer to the distribution manifold.
In the present invention, the duty of the pre-reformer is preferably from about 5 to about 20 percent of the duty of the radiant section. The transition section and pre-reformer are preferably a modular unit. The pre-reformer tubes preferably have an inside diameter of at least about 125 mm. A pressure drop through the pre-reformer tubes is preferably less than about 0.1 MPa. The catalyst in the catalyst-filled tubes, at least in the radiant section, preferably has a substantially uniform size distribution. The catalyst in the catalyst-filled tubes in the pre-reformer preferably has a nickel content from about 15 to about 19 weight percent and is optionally promoted with potassium.
The process or method described above can further include the steps of preheating combustion air for the firing of the radiant section by passing at least a portion thereof through an air preheat coil in the convection section; and in response to any variations in catalyst activity in the pre-reformer, adjusting the air preheat temperature, the radiant section firing rate or a combination thereof, for process control. The process can also include using the pre-reformer as a guard for the catalyst in the radiant section, whereby the partially reformed hydrocarbon feed stream contains a sufficient amount of hydrogen so as to substantially avoid coke formation in the radiant section tubes.